Intensive cropping of maize in the Southeastern United States

SOIL TECHNOLOGY ELSEV I ER

Soil Technology 7 (1994) 197-208

Intensive cropping of maize in the Southeastern United States W.J. Busscher a'*, R.E. Sojka b aCoastal Plains Soil, Water, and Plant Research Center, USDA-ARS, P.O. Box 3039, Florence, SC 29950-3039, USA bSnake River Soil and Water Management Research Unit, USDA-ARS Route 1, Box 3793N. 3600E., Kimberly, ID 83341, USA

Abstract

The long growing season of the southeastern Coastal Plains allows planting of a second crop after spring-planted maize (Zea mays L.). Second crops have been shown to reduce erosion and prevent leaching of nutrients and pesticides. Maize grown with a second annual crop might also have a yield advantage over mono-cultured maize. Seven tillage/cropping systems were compared. They included disking for weed control, disking for seedbed preparation, or no disking. Double-cropped treatments included sunflower (Helianthus annuus L. ), soybean ( Glycine max. L. ), a cover crop [ crimson clover (Trifolium incarnatum L.)] or no double crop. Double-cropped soybean yields did not respond to irrigation. They averaged 0.63 Mg/ha over 4 years. This is less than half of the local non-doublecropped yields. Sunflower yields averaged 0.89 Mg/ha, also less than non-double-cropped yields ( 1.0-2.5 Mg/ha). The best continuous maize yields (7-8 Mg/ha) were from treatments with disking in some phase of the operation. Treatments with lower maize yields generally had higher plant nutrient contents. Double-cropped maize yields significantly (P < 0.10) outyielded mono-cropped maize yields in two of the three years. In 1984, a dry year, the minimum tillage treatment had lower tensiometer readings than the conventionally tilled treatment. Keywords: Maize, intensive cropping; Southeastern United States, maize

1. I n t r o d u c t i o n

Maize continues to be an economically attractive crop in the southeastern Coastal Plains of the United States because of known production practices and ease of marketing. Recommendations for continuous maize have been avoided because of declining yields over *Corresponding author.

1994 Elsevier Science B.V. SSDIO933-3630(94)OOOO6-P

198

w.z Busscher, R.E. Sojka ~Soil Technology 7 (1994) 197-208

time without rotation, measured in other parts of the country (Havlin et al., 1990; Crookston et al., 1991; Martin et al., 1991; Johnson et al., 1992). The long growing season in the southeast, however, offers the possibility of interrupting the maize sequence with alternate crops in the same year. The effect of such a practice on the primary maize crop for these conditions is unknown. Most of the region has an average frost free growing season approaching 300 days (US Dept. of Commerce, 1968). One can harvest maize in early to mid August and can expect frost free temperatures until mid-November. Temperatures during this period range from normal daily minimums of 21 °C (70 °F) in August and 4 °C (40 °F) in November to normal daily maximums of 32 °C (90 °F) in August and 20 °C (68 °F) in November. Various fall crops have the potential for economic return (Sojka et al., 1990), for nitrogen production (Ebelhar et al., 1984), or as a conservation crop to hold soil and prevent leaching of pesticides and fertilizers (Zhu et al., 1989). Rainfall distribution favors fall cropping with 30% of the 1100 mm coming during August to November (NOAA, 1983). Soils are generally sandy, however, and low in water holding capacity. They can retain as little as 75 mm of water per meter of soil (Beale et al., 1966). The soils may enter the second cropping sequence dry because of water extraction by the maize. Conversely, when winter tillage is not performed or when winter cover crops or weeds are not killed several weeks before maize planting, spring crops have been shown to suffer from preseason profile depletion (Campbell et al., 1984a). One soil and water conservation practice is to reduce surface tillage. No-tillage is not feasible in many Coastal Plain soils because of a subsurface root restricting hardpan (Busscher et al., 1986). However, in-row subsoiling can disturb as little as 7 cm around the row while still breaking the hardpan enough to permit root growth through it and into the subsoil below (Busscher et al., 1988). In-row subsoiling is equally effective in disrupting the subsurface hardpan either with or without the surface tillage (Busscher and Sojka, 1987). Reduced surface tillage and increased soil cover have reduced erosion (Langdale et al., 1979) and increased infiltration (Mills et al., 1988). This study sought to determine how production of a second crop within a growing season would effect maize compared to mono-cropping. The study included several alternative management systems some of which included disking. All management systems used their appropriate pest and weed control measures.

2. Methods In the spring of 1982 a field of Norfolk loamy sand (fine, loamy, siliceous, thermic, Typic Kandiudult) near Florence, SC, USA, was planted to maize ( Z e a mays L. cv Pioneer 3572 l). The field design was four blocks split on water management (irrigation and rainfed). Field preparation in the spring of 1982 included three diskings of the soybean stubble (using a 2-m wide disk with 0.46-m diameter fluted coulters on the front gang and 0.53-m diameter smooth-edged coulters on the rear gang - Deere & Co., Moline, IL, USA). After disking, ~Mentionof trademark,proprietaryproduct, or vendordoes not constitute a guaranteeor warrantyof the product by the U.S. Dept. of Agric. and does not implyits approvalto the exclusionof other products or vendorsthat may also be suitable.

W.J. Busscher, R.E. Sojka ~Soil Technology 7 (1994) 197-208

199

the field was smoothed with a King Field Conditioner (King Plow Co., Atlanta, GA, USA), a tined field cultivator with rolling baskets. In the fall of 1982 we randomized seven treatments, described in Table 1, within the blocks. Maize was grown continuously, without a second crop, in conventional tillage (No. 1), reduced tillage (No. 3), and minimum tillage (No. 4) treatments (Table 1). Treatment No. 1 was disked both at maize seedbed preparation and for winter weed control. Treatment No. 3 was disked only at maize seedbed preparation. Treatment No. 4 was not disked. Subsoiling, as part of the subsoil-planting, was common to all treatments and was the only tillage applied to treatment No. 4. Maize, hybrid Pioneer 3572, was planted in 1982 and 1983. Because of seed availability, we changed to hybrid Pioneer 3950 in 1984 and 1985. Lime (1100 kg/ha per year) and fertilizer (Table 2) were applied according to soil tests (Clemson University, 1982). Maize Table 1 Treatment factors Treatment

1 2 3 4 5 6 7

2nd Crop

none interseeded soybean none none clover cover crop drilled soybean sunflower

Seedbed preparation

F a l l / w i n t e r weed control 1

2nd Crop

Maize

2nd Crop

Winter 2

none

disking no-till

chemical ~

disking chemical

none

disking no-till no-till

-

chemical chemical cover crop

none

no-till

chemical 4

chemical

disking

no-till

chemical ~

chemical

1Terbufos or carbofuran banded at maize planting for all treatments. 2Disking/chemicals (paraquat or glyphosate) were used as needed. 3Acifluorfen or sethoxydim (post-emergence). 4Glyphosate, alachor, and metribuzin (pre-emergence). 5Chloramben (pre-emergence). Table 2 Fetilizer applied ( k g / h a ) Year

Application

Treatment

N

P205

K20

1982 1983

Broadcast Broadcast Sidedressed Sidedressed Broadcast Sidedressed Sidedressed Broadcast Sidedressed Sidedressed

all all all 7 all all 7 all all 7

200 35 175 235 35 135 235 35 120 200

16 70

35 200

85 70

170 200

85 70

170 200

85

85

1984

1985

200

w.J. Busscher, R.E. Sojka~Soil Technology 7 (1994) 197-208

was in-row subsoil-planted on 0.75-m row centers using a Brown-Harden Super Seeder (Brown Manufacturing Corp., Ozark, AL, USA). This tillage tool subsoiled about 0.45-m deep in line with and ahead of John Deere 71 flexi-planters (Deere & Co., Moline, IL, USA) in a single, integrated operation. Maize subsoil-planting for all treatments was identical. We banded insecticides terbufos (Counter 15G, 2.25 kg A I / h a ) or carbofuran (Furadan 15G, 2.25 kg A I / h a ) in the row above the seed at maize planting each year. Plots were 6 rows wide by 30-m long and were randomly split into 15-m long irrigated and rainfed subplots. Subplots were irrigated with inverted microirrigation tubes placed between the rows, operated at 80 kPa pressure. In each irrigated plot we installed a gagetype tensiometer (Soilmoisture Equipment Corp., Santa Barbara, CA, USA) at 0.30-m depth and read them three times a week. When tensions in any tensiometer exceeded 40 kPa, the maize was irrigated (Table 3). In 1984 and 1985, banks of tensiometers were installed in both the irrigated and rainfed splits of treatments No. 1 and No. 4 at 0.15-m, 0.3-m, 0.6-m, 0.9-m, 1.2-m, and 1.5-m depths. Banks were installed in both in-row and mid-row positions in reps 2, 3, and 4 in 1984 and all four reps in 1985. These tensiometers were read 2 to 3 times a week for approximately fifty days from June 15, 1984 and June 7, 1985. On June 12, 1984 we took maize ear leaf samples and on May 13, 1985 whole plant samples. Plant samples were only taken from the irrigated split. The Clemson University Plant Tissue Lab analyzed the plant samples for Kjeldahl nitrogen, phosphorous, and potassium (Clemson University, 1982). Treatments No. 2, 5, 6, and 7 (Table 4) had a second crop: a cover crop or a doublecrop. Treatment No. 5 had a crimson clover (Trifolium incarnatum L. cv Tibbee) cover crop seeded at 6.8 kg/ha. In 1982, 1983, and 1984 we hand planted the clover cover with a Unico (Universal Coop Products, Minneapolis, MN, USA) hand-carried rotary seed sower. In 1985 we drilled the clover cover crop with the KMC Unidrill (Kelly Manufacturing Co., Tifton, GA, USA) with disk openers. Treatment No. 2 had a maturity group VIII soybean (Glycine max L. cv Cobb) second crop. It was interseeded into the maize Table 3 Rainfall/irrigation (mm) Year

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Total

1982

117

119

29

104

87

151 /57

141 /25

61

105 /34

92

35

135

1176

1983

109

169

236

42

60

66 /50

97 /38

59 /25

85 /39

74

91

164

1252 /152

1984

70

102

140

90

146 /31

28 /82

343

61 /25

20 /50

30

8

37

1075 /188

1985

119

107

26

22 /50

54 /44

148

194

128

101 /38

29

170

17

80

78

108

100

66

22 Yr Mean

83

90

103

150

120

1115

/132 51

69

1098

201

W.J. Busscher, R.E. Sojka ~Soil Technology 7 (1994) 197-208

Table4 Planting and harvestdates (month/day) Crop

Treatment

Maize

All

Plant Harvest

3/23 8/3

Soybean

2

Plant Harvest Plant Harvest

7/27 11/18 8/6 11/24

7/20 1/3/84 8/17 1/3/84

6

1982

1983 3/16 8/15

1984

1985

4/2 8/13

4/1 8/12

7/19 12/10 8/16 12/10

7/16 12/10 8/14 12/10

Clover

5

Plant

10/1

9/16

9/18

9/17

Sunflower

7

Plant Harvest

8/10 11/ 12

8/18 11/26

8/17 11/8

8/14 11/27

(Table 4) with the hand planter. Treatment No. 6 also had a soybean second crop. Treatment No. 7 had a sunflower (Helianthus annuus L. cv DO-844, MCF610, and Sheyenne 24906 in 1982, 1983, and 1984 and IS-7000 in 1985) second crop. Both treatments No. 6 and No. 7 were seeded (Table 4) using the grain drill with every other seed opener closed giving 0.33-m row spacings. Bird damage in the sunflower plots was visually estimated and data were corrected before analysis as reported in Sojka et al. (1989). All maize and sunflower were over-planted and thinned to approximately 75000 (irrigated) and 50000 (rainfed) plants/ha 7 to 10 days after emergence in 1983, 86500 (irrigated) and 61800 (rainfed) plants/ha in 1984, and 96400 (irrigated) and 86500 (rainfed) plants/ha in 1985. Soybean treatments were planted in 0.33-m row spacings at a population of about 470000 plants/ha. Table 4 lists the planting and harvest dates for maize and for second crops. Maize, soybean, and sunflower yields were analyzed as a randomized complete block model with no split, split, or split-split plot designs. Treatments were the main effect. Irrigation and year were the splits (Table 1 ). Sunflower hybrid effects were significantly different only in 1984; they were averaged before final analysis. For maize, there were year by treatment interactions but no irrigation by treatment interactions. Therefore, we reanalyzed the maize data by year. Maize nutrient content was analyzed by year with a simple analysis of variance. Tensiometer data was analyzed using treatment as the main effect and irrigation, positions (mid-row and in-row), depth, and date of measurement as splits. For all analyses, we used the GLM procedure of SAS (SAS Institute, 1990) with a least significant difference mean separation procedure. Single-degree-of-freedom contrast statements were also used to compare specific groups of treatments. We considered differences up to P < 0 . 1 0 . 3. Results and Discussion 3.1. Maize yield

In 1982, before the treatments were applied to the plots, maize yields were 12.7 M g / h a for the irrigated and 10.1 M g / h a for the rainfed treatments. This was 1.4 to 1.6 times greater

202

w.J. Busscher, R.E. Sojka~Soil Technology 7 (1994) 197-208

than the yields for 1983 through 1985 (Table 5). The 1982 maize yields were generally higher than normal for the Pee Dee region of South Carolina (5.5 M g / h a : USDA, 1990). Others (Karlen and Sojka, 1985) produced extraordinarily high yields in this area in 1982 as well. The years 1980 and 1981 were drought years. Despite soil testing, some residual fertilizer was probably available below the sampling depth following the drought years, as documented in other studies (Campbell et al., 1984a). Also, though the total annual rainfall for 1982 was not different from that for 1983 to 1985, it was more uniform during the 1982 maize growing season (Table 3). Temporal uniformity of rainfall is especially important for the Norfolk soil that is sandy and has a low water holding capacity (Beale et al., 1966).

All three years For the years 1983 through 1985, irrigated maize yields, 7.99 M g / h a , were significantly ( P < 0.01 ) higher than rainfed, 6.51 M g / h a (Table 5). Maize yields were also significantly ( P < 0.10) different among years with 1983 having a higher mean yield than 1984 and 1985. The higher yield of 1983 was probably a result of an earlier planting date (Table 4) following a higher winter rainfall than 1984 and 1985 (Table 3). The high winter rainfall, especially the 236 m m in March of 1983, at planting, is important because the winter weeds can desiccate the low-water-holding-capacity Norfolk soil.

By year Since there was a significant ( P < 0.02) year by treatment interaction, we reanalyzed the maize yield data by year. For the analysis by year, irrigated maize plots had 20% to 26% higher yields than rainfed plots (Table 5). This difference was significant at P < 0.01.

Table 5 Maize yield (Mg/ha) at 15% moisture content for the seven managementtreatments Treatment 1983

1 2 3 4 5 6 7 Means2

1984

1985

Irrigated1 Rainfed

Mean

Irrigated Rainfed

Mean

IrrigatedRainfed Mean

8.65ab 8.06bc 8.75ab 7.49c 6.63d 8.99a 8.26abc 8.12a

7.79a 7.46ab 7.96a 6.97bc 6.68c 7.66a 7.52ab

8.32a 8.21a 8.40a 8.29a 6.40c 8.05ab 7.31b 7.85a

7.62a 7.61a 7.71a 7.32a 6.04c 7.20a 6.58b

8.26ab 8.5lab 7.60bc 6.81c 7.44bc 8.54ab 8.86a 8.00a

6.94ab 6.86ab 7.16a 6.44ab 6.74ab 6.33b 6.78ab 6.75b

7.43a

6.91abc 7.00ab 7.02a 6.34cd 5.69e 6.35bcd 5.84de 6.45b

7.15b

7.10a 6.8lab 6.27b 6.15b 5.01c 6.57ab 6.36b 6.33b

7.68a 7.66a 6.93ab 6.48b 6.23b 7.56a 7.61a 7.16b

~Means with the same letter within a column are not significantlydifferent treatments at P< 0.10 using the lsd mean separation. 2Meansby irrigationby year and by year with the same letter within a row are not significantlydifferenttreatments at P <0.10 using the lsd mean separation.

W.J. Busscher, R.E. Sojka ~Soil Technology 7 (1994) 197-208

203

Disked vs. non-disked

For mono-cropped maize, the conventional tillage, No. 1, and reduced tillage, No. 3, treatments outyielded the minimum tillage treatment, No. 4. Treatments No. 1 and No. 3 were compared with No. 4 using a contrast in the analysis of variance. Treatments No. 1 and No. 3 had significantly (P < 0.05) higher yield in 1983 and 1985. This showed a yield advantage for the disked treatments, the conventional (No. 1) and the reduced tillage treatments (No. 3), over the minimum tillage treatment (No. 4). Neither treatment No. 1 nor No. 3 were consistently higher than the other. Both treatments No. 1 and No. 3 were disked at seedbed preparation for the maize. The overall advantage of disking for maize yield was partially verified by contrasting plots that had some disking in their management scheme (treatments No. 1, 3, and 7) with those without any disking (treatments No. 2, 4, 5, and 6). In 1983 and 1985, disked plots had significantly higher (P < 0.10) maize yields. In 1984, disked plots had higher maize yields but the difference was not significant. Yield difference between disked and non-disked treatments is usually attributed to better seed soil contact in the disked treatments and poorer stand establishment in the non-disked treatments (Campbell et al., 1984a; Karlen and Sojka, 1985). Non-disked stand counts, measured at harvest, were on the average 4.5% lower than disked stand for all three years (Table 6). Differences between treatment No. 1 (no second crop, disked) and treatment No. 4 (no second crop, not disked) were significant (P < 0.10) for 1984 and 1985. Though not quantified, maize in residue were also less vigorous and less uniform in size. We used the same planter for all treatments. It was a conventional-tillage planter. Different planters for the different surface conditions would have complicated the experiment. However, heavier planters used in reduced tillage plots can give more uniform, more vigorous stands. Double cropped

Among the treatments with a double crop (No. 2, 6, and 7), maize yield for treatment No. 2 (interseeded soybeans) was significantly (P < 0.10) higher than treatment No. 7 (drilled sunflower) in 1984. However, this difference, and other differences, were not Table 6 Maize stand count (plants/ha × 104) at the end of the year for the seven management treatments Treatment 1983

1 2 3 4 5 6 7 Means 2

1984

1985

Irrigated I Rainfed

Mean

Irrigated

Rainfed

Mean

Irrigated

Rainfed

Mean 2

5.47bc 5.27c 5.72bc 5.52bc 4.30d 6.62a 5.94b 5.55a

5.04ab 4.94b 5.14ab 5.05ab 4.20c 5.50a 5.22ab

7.09ab 6.82abc 7.19a 6.09d 6.42cd 6.52bcd 6.48cd 6.66a

5.76ab 6.21a 5.80ab 5.14c 5.77ab 5.26c 5.41bc 5.62b

6.43ab 6.51a 6.50a 5.61c 6.09abc 5.89c 5.94bc

8.61a 8.23ab 7.29abc 6.46c 7.13bc 8.15ab 8.10ab 7.71a

6.48a 6.59a 6.00a 6.32a 5.68a 6.13a 6.59a 6.26b

7.54a 7.4lab 6.64ab 6.39b 6.40b 7.14ab 7.34ab

4.61a 4.61a 4.55a 4.57a 4.09a 4.38a 4.50a 4.47b

~Means with the same letter within a column are not significantly different treatments at P > 0.10 using the lsd mean separation. 2Means by irrigation by year with the same letter within a row are not significantly different treatments at P > O.10 using the lsd mean separation.

204

w.J. Busscher, R.E. Sojka / Soil Technology 7 (1994) 197-208

consistent over the three years. Specifically, soybean seeding method (treatments No. 2, interseeded and No. 6, drilled) did not affect maize yield. Maize yields for the clover winter cover, treatment No. 5, were the lowest overall and were significantly (P < 0.10) lower than for the double-cropped treatments, No. 2, 6, and 7, for the irrigated subplots in 1983. When we compared treatment No. 5 to treatments No. 2, 6, and 7 in a contrast statement, maize yields were significantly lower (P < 0.05) for all three years for the irrigated subplots and for 1984 and 1985 for the rainfed subplots. Lower yields for fields with cover crops in this area are usually explained by plant water use of the cover crop and subsequent poor seedling growth caused in dry seedbeds (Campbell et al., 1984a, b). In treatment No. 5, maize was planted into the clover just before spraying it with paraquat. In this sandy soil, killing the cover crop early enough to allow sufficient rewetting of the profile is important for proper early season growth (Campbell et al. 1984a, b; Ewing et al., 1991). This may have caused the differences seen here. We partially verified this with maize stand count at harvest. Stand counts for treatment No. 5 were lower than the double-cropped treatments by 24% in 1983, 0.4% in 1984, and 14% in 1985, though the treatments had been thinned to the same population. These differences were only significant ( P < 0 . 0 1 ) in 1983. Mono-cropped vs. double-cropped

We contrasted mono-cropped maize, treatment No. 4, to similar double-cropped treatments, No. 2, 6, and 7. Double-cropped treatments had significantly ( P < 0 . 0 3 ) higher maize yields in 1983 and 1985. The differences, and the significances, were mainly for the irrigated subplots where the double-cropped treatments were 13 to 27% higher than the mono-cropped treatment. The rainfed treatments followed similar trends but were not significant in any year when they were analyzed alone. 3.2. Tensiometer readings

Tensiometers were placed in treatments No. 1 and 4 to compare water contents of conventional and minimum tillage. Tensiometer readings were always lower for the irrigated splits (P < 0.05). There was no significant difference between the tensiometer readings at the in-row and mid-row positions. The driest part of the profile (P < 0.05) was at about the 0.6-m to 0.9-m depths (Tables 7 and 8) for both the irrigated and rainfed splits. This corresponds to the upper part of the B horizon where roots that have grown through the subsoiled hardpan would have extracted water (Busscher et al., 1988). Conditions were dry in 1984, except for a two-week period in July (Table 3). In 1984, the minimum tillage treatment (No. 4) was wetter than the conventional tillage treatment (No. 1). In 1985, a year of relatively uniform rainfall distribution, neither treatment was consistently wetter than the other. 3.3. Nutrient analysis

The earlier sampling date and younger plants resulted in a higher plant nutrient analysis for the whole plant samples of 1985 than for the ear leaf samples of 1984 (Table 9). Higher yielding maize treatments generally had lower nutrient analyses. Using a contrast statement,

205

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Table 7 Tensiometer readings (kPa) for 1984. Each number is the men of three ~ reps and 21 readings from June 16 to August 8 Depth (m)

0.15 0.3 0.6 0.9 1.2 1.5 mean

Irrigated treatment

Rainfed treatment

I

4

mean 2

1

4

mean

26 65 86 83 48 35 57a

23 21 66 42 28 19 33b

25c 43bc 76a 63ab 39c 27c

100 195 139 195 117 91 140a

131 112 130 147 80 57 109a

ll6abc 153ab 135ab 171a 98bc 74c

~Rep 1 did not have tensiometers in 1984. 2Means within treatment with the same letter ar not different at P < 0.05 using the lsd mean separation. Table 8 Tensiometer readings (kPa) for 1985 Depth (m)

0.15 0.3 0.6 0.9 1.2 1.5 mean

Irrigated treatment

Rainfed treatment

1

4

Mean I

1

4

Mean

52 137 280 202 150 122 157a

54 56 262 208 149 91 137a

53e 96d 271a 205b 149c 107d

233 277 341 341 250 152 266a

224 330 415 354 266 174 294a

228d 303bc 378a 348ab 258cd 163e

Each number is the mean of four reps and thirteen readings from June 7 to July 19. ~Means with treatment with the same letter are not different at P < 0.05 using the lsd mean separation. the higher y i e l d i n g disked treatments ( N o . 1, 3, 7) had l o w e r N and P in 1984 and N, P, and K in 1985 than the n o n - d i s k e d treatments ( N o . 2, 4, 6) at P < 0 . 0 3 . The s a m e pattern was seen with the n o n - d o u b l e c r o p p e d treatments ( N o . 1, 3 vs. No. 4 ) , but differences were not significant in 1984. T h e l o w e s t yielding treatment (No. 5, the c l o v e r c o v e r crop) had higher N, P, and K in 1984 and P and K in 1985 than the other treatments though it was not always significantly higher ( T a b l e 9). H i g h nutrient content o f the l o w e r yielding treatments w o u l d be an indication o f adequate nutrient availability with s l o w growth and water stress (though the treatments were the irrigated). T h e water stress can be partially explained by not having irrigation on the plots from the b e g i n n i n g o f the g r o w i n g season c o m b i n e d with the low water holding capacity o f the soils. Microirrigation tubes w e r e installed by June 14, 1983, M a y 23, 1984, and April 25, 1 9 8 5 : 3 to 13 w e e k s after planting. In contrasts, the irrigated d o u b l e - c r o p p e d treatments ( N o . 2, 6, and 7) had higher maize yields than irrigated m o n o - c r o p p e d m a i z e treatment No. 4 in 1985. H o w e v e r , m a i z e N, P,

206

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Table 9 Nutrient analysis (g/Kg) of the irrigated subplots for June 12, 1984 (ear leaf samples) and May 13, 1985 (whole plant samples) Treatment

1 2 3 4 5 6 7

1984

1985

N

P

K

N

P

K

26. I a ~ 25.8a 25.9a 26. la 27.8a 26.6a 23.5b

2.72c 3.08bc 3.00bc 3.15ab 3.45a 3.05bc 3.10ab

30.6ab 28.9bc 28.8bc 28.9bc 31.5a 27.4c 28.4bc

33.3b 39.6a 33.5b 38.9a 40.1 a 40.5a 34.8b

2.91 c 3.80a 3.19bc 3.69a 3.90a 3.64a 3.56ab

52.0cd 59.0ab 51.6d 57.8ab 61.0a 59. Iab 56.0bc

~Means with the same letter within the column are not signficantlydifferent treatments at P > 0.05 using the lsd mean separation. and K values were not significantly different; they were similar (Table 9). This would be consistent with recent findings by Johnson et al. (1992) that soil fungi in monoculture are detrimental to the uptake o f nutrients but not in rotations. They found that soil micorrhizal fungi were negatively correlated with yield and tissue mineral concentration of continuous maize. They suggested that the buildup of fungi, to the detriment of maize nutrient uptake, caused a yield decrease. Treatments No. 5 and 6 (clover cover crop and drilled soybean) had the highest N contents in 1984 and 1985. This is consistent with their ability to fix soil nitrogen. To further verify this, a contrast of legume treatments No. 2, 5, and 6 vs. non-legume treatments showed significantly ( P < 0.05) higher N in both 1984 and 1985.

3.4. Double-crop yields

Table 10 lists the yields for the double crops, soybean and sunflower. Neither the soybean nor the sunflower had yields that were consistently high enough to be on a par with monocropped culture. Soybean yield averaged 0.63 M g / h a over 4 y and sunflower yields averaged 0.89 M g / h a . Soybean yields did not differ between the drilled and interseeded treatments or between irrigated and rainfed treatments. There were yield differences among years. In 1985, soybean yield was higher ( P < 0.05) because of an especially wet late summer and early fall (Table 3). In that year, soybeans yields, averaging 1.19 M g / h a , were comparable to county average full-season yields of 1.3 M g / h a ( U S D A , 1990). Net sunflower yields in 1982 and 1984 were toward the bottom of the range of mono-cropped production, 1.0 to 2.5 M g / h a (Sojka et al., 1990). This is at least partly attributed to sunflower high fertilizer requirement (Table 2).

W.J. Busscher, R.E. Sojka ~Soil Technology 7 (1994) 197-208

207

Table 10 Double crop yield (kg/ha) for soybean at 13% moisture content and for sunflower at 9% moisture content Crop

Treatment ~

Soybean

2 6 2&6

Sunflower

7

I N I N Mean I N Mean

1982 265 450 638 418 443bc 1193 1144 1168a

1983

1984

1985

573 852 420 359 551b

457 281 450 230 354c

959 793 1678 1334 l191a

253 349 301c

1006 1178 1092ab

815 1178 997b

~I = irrigated, N = Rainfed, treatment means in the same row with the same letter are not statistically significant different at P < 0.05 using the lsd mean separation.

4. Conclusion Of the seven management treatments in the experiment, those with disking in some phase of the management had best maize yields. Better seed-soil contact of heavier new improved planters could improve the non-disked stand and yield. Averaged over the four years neither second crop produced yields that were on a par with the full season yields. However, including a second crop increased maize yields for two of the three years. Lower yielding treatments generally had higher plant nutrient contents. In 1985, maize yields for the doublecropped treatments were higher than mono-cropped maize yields but nutrient concentrations were not significantly lower. This is consistent with recent findings by Johnson et al. (1992) that soil fungi in monoculture are detrimental to the uptake of nutrients but not in rotations. Successful adaptation of this system in this or other areas would depend on the system's ability to improve maize and double-crop production and on the conservation aspects of the double crop.

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